A) Field of the Invention
This invention relates to a semiconductor light emitting device and its manufacture.
B) Description of the Related Art
Light emitting diode (LED) utilizing nitride semiconductor, such as GaN, can emit ultraviolet light or blue light, and can emit white light by utilizing fluorescent material. LEDs capable of emitting white light of high output can be used for illumination purposes.
Recently, with the increase in the output of semiconductor light emitting devices, device structures in which semiconductor lamination is attached to and supported by a heat-dissipating and supporting substrate in view of providing electrodes on both surfaces, etc. are employed. For realizing high output, a light reflecting electrode formed of silver (Ag) or Ag alloy which contains not more than 1% of selenium (Se), palladium (Pd), gold (Au), copper (Cu), platinum (Pt), neodymium (Nd), bismuth (Bi), etc. mixed in Ag is disposed between the supporting substrate and the semiconductor lamination.
It is known that Ag has high light reflectivity, but has such property that migration, i.e. movement, of Ag ions is easy to occur, especially when a current is flowing or a temperature is raised. When migration of Ag ions occurs, phenomena that give affects to the characteristics and reliability of the device, such as leakage current, may occur.
For preventing migration of Ag ions, such countermeasures as formation of diffusion preventing layer made of high melting point metal such as Ti, Pt, or metal oxide such as indium tin oxide (ITO) is widely employed. For example, a transparent electrode of metal oxide such as ITO is formed on a nitride semiconductor layer, a Ag-containing reflecting electrode is formed on the transparent electrode, a diffusion preventing film including lamination of Ti and Pt is formed to cover the Ag-containing reflecting electrode, and a thick Au film is formed thereon (for example, see JP-A 2008-192782).